We shall attempt to determine if tumor growth is associated with an increase in the production and concentration of an immunosuppressive factor which is normally found in very low concentration in the serum. We shall develop an immunoassay for this human serum factor and shall develop other tissue culture assays for the murine factor in order to have an accurate quantitative measure of this peptide. We shall measure concentrations of this factor in human patients bearing a variety of cancers. We shall correlate the blood levels of this factor with the presence, type, and mass of tumor and with tumor therapy. In mice we shall correlate blood levels of this factor with type of cancer, growth rate, and with the ability of the mice to develop or express tumor immunity. Most important, we shall determine if we can remove this factor from the serum of tumor bearing mice by treatment with antibody; and we shall determine if this treatment will result in an increase in tumor immunity.